Priority is claimed from Japanese Patent Application No. 067901/2004, filed Mar. 10, 2004, the content of which is incorporated herein by reference.
1. Field of Invention
The present invention relates to a mobile communication system, and in particular, to transmission and reception diversity technologies.
2. Description of the Related Art
FIG. 14 shows a wireless communication system in which a receiving apparatus 05 receives wireless signals from a transmitting apparatus 01 and/or 02 by means of an antenna 04. In the event of using only one antenna to receive signals, as shown in FIG. 14, there may be interference between signals, and thus the receiving power may be significantly reduced. Specifically, interference may occur between a direct wave a (or b) which is an electric wave transmitted from apparatus 01 (or 02) having directly reached the receiving antenna 04 and a reflected wave a′ (or b′) which is the electric wave from the transmitting apparatus 01 (or 02) having been reflected off an object 03 and having reached the receiving antenna 04. According to the system of FIG. 14, the use of only a single antenna may result in poor reception quality. Therefore, it is desirable for a receiving apparatus in a mobile communication system to have multiple antennas.
Japanese Laid-Open Patent No. 2003-32162 (JP '162) discloses a technique of antenna diversity reception, applicable to a UWB (Ultra-Wide Band) transmission system. According to JP '162, when a signal from a single transmission apparatus is being received at a reception apparatus, antenna diversity is obtained by determining which of a plurality of antennas receives the best reception and by selecting that antenna through which the signal from the transmission apparatus is received. According to this system, in order to obtain the best reception quality, a receiving apparatus can select the antenna which receives the best reception from a plurality of antennas. However, this system works well only if the receiving apparatus is receiving only a single signal at a time from a single transmission apparatus. If, on the other hand, a receiving apparatus is receiving a plurality of signals from a plurality of transmission apparatuses, the reception quality should be optimized with respect to all of the signals being received. However, such an optimization can not be obtained by merely selecting one of a plurality of antennas based on the reception quality of a single signal, because the plurality of signals from the plurality of transmission apparatuses are received via numerous signal paths and the best antenna may be different for each signal.
In addition to the above, and also in order to obtain a diversity gain (i.e. to obtain the best possible signal), JP '162 also describes a mobile communication base station which is equipped with two antennas and a separate reception circuit connected to each antenna. However, in order to reduce size and costs, it is desirable that a reception apparatus have only a single reception circuit even if the apparatus is equipped with multiple antennas.